Recently, it is required for resist materials to have, as important characteristics, superior forming performance of resist patterns, which become finer year after year. For fulfilling the request, star polymers have been proposed as one of the materials. Typical among them are alkenylphenolic star polymers, and for example, star polymers having divinylbenzene polymers as a core part and p-hydroxystyrene-styrene copolymers, p-hydroxystyrene-butadiene copolymers or the like as an arm part have been disclosed (Patent Document 1).
Furthermore, as an alkenylphenolic star polymer having a core part other than divinylbenzene polymers, (meth)acrylic acid star polymers including polymer chains containing repeating units derived from di(meth)acrylates as a core part, and polymer chains containing repeating units derived from alkenylphenols and repeating units derived from acrylic acid ester derivatives of alicyclic hydrocarbon groups (additional repeating units derived from alkenylphenyls may be contained) as an arm part have been proposed (Patent Document 2).
However, there is much room for improvement in sensitivity and resolution, for example, in techniques for forming resist patterns with the star polymers ever proposed. For example, the star polymers of Patent Document 2 are acid-degradable because their core parts are formed from poly(meth)acrylate derivatives, so that the polymers are expected to have superior sensitivity when used as resist materials, but the polymers still have problems from the viewpoint of resolution, for example.